Re: [PATCH] mm/compaction: count pages and stop correctly during page isolation.

[Yang Shi]

On Mon, Nov 2, 2020 at 5:03 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 10/30/20 7:55 PM, Yang Shi wrote:
> > On Fri, Oct 30, 2020 at 11:39 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
> >>
> >> On 30 Oct 2020, at 14:33, Yang Shi wrote:
> >>
> >> > On Fri, Oct 30, 2020 at 6:36 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >> >>
> >> >> On Fri 30-10-20 08:20:50, Zi Yan wrote:
> >> >>> On 30 Oct 2020, at 5:43, Michal Hocko wrote:
> >> >>>
> >> >>>> [Cc Vlastimil]
> >> >>>>
> >> >>>> On Thu 29-10-20 16:04:35, Zi Yan wrote:
> >> >>>>> From: Zi Yan <ziy@xxxxxxxxxx>
> >> >>>>>
> >> >>>>> In isolate_migratepages_block, when cc->alloc_contig is true, we are
> >> >>>>> able to isolate compound pages, nr_migratepages and nr_isolated did not
> >> >>>>> count compound pages correctly, causing us to isolate more pages than we
> >> >>>>> thought. Use thp_nr_pages to count pages. Otherwise, we might be trapped
> >> >>>>> in too_many_isolated while loop, since the actual isolated pages can go
> >> >>>>> up to COMPACT_CLUSTER_MAX*512=16384, where COMPACT_CLUSTER_MAX is 32,
> >> >>>>> since we stop isolation after cc->nr_migratepages reaches to
> >> >>>>> COMPACT_CLUSTER_MAX.
> >> >>>>>
> >> >>>>> In addition, after we fix the issue above, cc->nr_migratepages could
> >> >>>>> never be equal to COMPACT_CLUSTER_MAX if compound pages are isolated,
> >> >>>>> thus page isolation could not stop as we intended. Change the isolation
> >> >>>>> stop condition to >=.
> >> >>>>>
> >> >>>>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> >> >>>>> ---
> >> >>>>>  mm/compaction.c | 8 ++++----
> >> >>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
> >> >>>>>
> >> >>>>> diff --git a/mm/compaction.c b/mm/compaction.c
> >> >>>>> index ee1f8439369e..0683a4999581 100644
> >> >>>>> --- a/mm/compaction.c
> >> >>>>> +++ b/mm/compaction.c
> >> >>>>> @@ -1012,8 +1012,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >> >>>>>
> >> >>>>>  isolate_success:
> >> >>>>>            list_add(&page->lru, &cc->migratepages);
> >> >>>>> -          cc->nr_migratepages++;
> >> >>>>> -          nr_isolated++;
> >> >>>>> +          cc->nr_migratepages += thp_nr_pages(page);
> >> >>>>> +          nr_isolated += thp_nr_pages(page);
> >> >>>>
> >> >>>> Does thp_nr_pages work for __PageMovable pages?
> >> >>>
> >> >>> Yes. It is the same as compound_nr() but compiled
> >> >>> to 1 when THP is not enabled.
> >> >>
> >> >> I am sorry but I do not follow. First of all the implementation of the
> >> >> two is different and also I was asking about __PageMovable which should
> >> >> never be THP IIRC. Can they be compound though?
> >> >
> >> > I have the same question, can they be compound? If they can be
> >> > compound, PageTransHuge() can't tell from THP and compound movable
> >> > page, right?
> >>
> >> Right. I have updated the patch and use compound_nr instead.
> >
> > Thanks. Actually I'm wondering what kind of movable page could be
> > compound. Any real examples?
>
> Looks like there's currently none. Compaction also wouldn't work properly with
> movable pages with order>0 as the free page scanner looks for order-0 pages
> only. But it won't hurt to use compound_nr() anyway.

Thanks, yes this is what I thought otherwise we have troubles in
migration path too.

>
> >>
> >> —
> >> Best Regards,
> >> Yan Zi
> >
>